1. Field of the Invention
The present invention relates to a projection zoom lens installed in a projection apparatus and the projection apparatus, and more particularly to a projection zoom lens used in a projection apparatus having a light valve, such as a DMD (digital micro mirror device) or the like, and the projection apparatus.
2. Description of the Related Art
A light valve is used in a projection apparatus (hereinafter, also referred to as “projector”) to perform light modulation for converting an image to a signal and a transmissive liquid crystal is known as one of the light valves.
A cross dichroic prism is generally used in an optical system in which a transmissive liquid crystal is used for color composition. In order to improve characteristics of color composition, a projection zoom lens used in the aforementioned optical system is telecentric on the reduction side.
Further, zoom lenses capable of changing image sizes are frequently used and the demand for zoom lenses having a high rate of changing, that is, a high zoom ratio has recently been increasing.
Japanese Unexamined Patent Publication No. 2008-186026 (Patent Document 1) is known as a document that discloses a projection zoom lens which is telecentric on the reduction side and has a high zoom ratio.
In the mean time, a projector having a DMD as the light valve is drawing wide attention with an increase in the demand for obtaining a projection image highly accurate and bright with a high contrast ratio.
A DMD includes a multitude of angle variable micro-mirrors and each pixel is switched from ON to OFF or vice versa by changing the angle of each micro-mirror and changing reflection directions of light emitted from the light source, whereby an entire projection image is formed.
In the case of a projection zoom lens using such DMD, known as a reflective light valve, the reduction side is not necessarily telecentric and the lens portion of the reduction side may be downsized.
Japanese Unexamined Patent Publication No. 2004-077950 (Patent Document 2), Japanese Unexamined Patent Publication No. 2004-240309 (Patent Document 3), and Japanese Unexamined Patent Publication No. 2007-333805 (Patent Document 4) are known as documents that disclose projection zoom lenses downsized in the lens portion of the reduction side as described above.
In the mean time, projectors used for business presentations or home theaters are generally required to be compact and wide angle as they are used in relatively small spaces and carried around without being installed permanently. In the case of an elongated space, however, the installation position of the projector is often fixed (permanently installed) at a place near the ceiling at the back and the projector is rather required to be telescopic in that case.
In the case where the installation position of a projector is changed every time the projector is used, a rough image size can be adjusted more easily as the projector has wider angle by changing the installation position rather than the zoom function. On the other hand, in the case where the installation position of a projector is fixed, the projection distance and image size are constrained, so that it is important for the zoom lens, to be a versatile lens, to have a high zoom ratio.
Further, in view of an application in which the projector is installed by suspending from the ceiling, it is important that the lens is downsized (lightweight) for reducing the weight of the entire projector
The projection zoom lenses described in Patent Documents 2 to 4 have zoom ratios around 1.2 to 1.3 that do not meet the demand for a high zoom ratio, though downsized.
Zoom lenses described in U.S. Pat. No. 5,448,412 (Patent Document 5) and Japanese Patent No. 4273556 (Patent Document 6) have high zoom ratios, but the zoom lens described in Patent Document 5 has a problem that it has large spherical aberration and astigmatism so that it is not suitable for use with the projection lens intended by the invention of the present application.
The zoom lens described in Patent Document 6 has a problem that it has a short back focus. It may be possible, as a means, to proportionally expand the overall lens system in order to obtain a desired back focus and to use a portion of the effective image height. But, the proportional expansion of the overall lens system may result in increased aberrations, as well as extended lens itself. Therefore, it is difficult to realize a size allowable as a projection lens and performance.